This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-287381, filed Sep. 21, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to a disk drive including a servo system and specifically to a servo system having a disk runout compensation function.
2. Description of the Related Art
A prior art disk drive such as a magnetic disk drive includes a servo system for locating a magnetic head (simply referred to as a head hereinafter) in a target position (target track or target cylinder) on a disk using servo data prerecorded on the disk. The servo system includes a feedback system using a microprocessor (CPU) as a main element, the microprocessor being a main controller of the disk drive.
The manufacturing process of a disk drive includes a servo-write step of recording servo data on a disk. In the servo-write step, a drive mechanism such as a head disk assembly is incorporated into the body of the drive and then servo data is recorded on the disk by a servo track writer (STW). As another method, servo data is prerecorded on the disk before the disk is incorporated into the drive body.
Servo data is recorded on a plurality of servo areas of the disk, which are arranged at regular intervals along the circumference of the disk. In each track, an area between adjacent servo areas is a data area. The data area is divided into a plurality of data sectors (access unit of data). A range from one servo area to its subsequent data sectors is called a servo sector. One track of the disk is a set of servo sectors for one round. There may be cases where a track defined by only servo areas is called a servo track and a track defined by a set of servo sectors is called a data track.
Thus, a number of tracks are formed concentrically on the disk. Such tracks may be called a cylinder under the concept that it includes tracks in the same position in the direction of rotation axis of one or a plurality of disks (the same position of the face and back when the number of disks is one). It is preferable that each of the tracks be formed as an almost perfect circle with reference to the rotation axis of the disk.
In the servo-write step using an STW, however, an external impact is given to the drive body and an influence is exerted upon the mechanical accuracy of a clamp mechanism for fixing the disk. These factors cause a phenomenon called a disk runout. The disk runout can be measured as an amount of rotation-runout or eccentricity that is caused in synchronization with the period of rotation of the disk. Due to the disk runout phenomenon, the servo tracks are not formed as an idealistic concentric circle but deformed with regard to the rotation center of the disk.
In the above disk drive, the servo system drives an actuator to locate the head in a target track on the disk when it reads/writes data. The servo system chiefly perform a seek operation for moving the head to a target track and a tracking operation for positioning the head within the range of a target track (usually on the center line of the track). In the tracking operation, the servo system follows servo data read out of the head. Actually, the servo system reads servo burst data contained in servo data and generates a positional-error signal. In response to the positional-error signal, the servo system adjusts the head onto the center line of the track. If, therefore, the servo data is greatly distorted due to a deformation of the servo track, the tracking operation becomes difficult to perform in normal feedback control.
Recent disk drives comprise a servo system having a learning function of calculating a disk runout component when a spindle motor for rotating the disk is started and performing servo compensation based on a result of the calculation (disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 11-353831). The servo system obtains a disk runout component (referred to as a learning value hereinafter) from a positional error of a head. The servo system calculates and stores a learning value for each head through a learning operation and executes servo compensation in a tracking operation using the learning value. More specifically, the servo system obtains a servo compensation value (correction value) for suppressing a disk runout from a learning value of each head calculated by the learning operation. The servo system adds the servo compensation value to a control value obtained by feedback control. This addition is performed by feed-forward control. If the servo system has a learning function, it can inhibit the head from being deformed in the tracking operation, thereby improving the accuracy of head positioning.
In a disk drive having a learning function, a learning value calculated by a learning operation performed when the disk drive is manufactured (hereinafter referred to as an initial learning value) is stored in a nonvolatile memory such as a flash ROM and an EEPROM. When a spindle motor is started, the servo system of the disk drive carries out a learning operation in which the initial learning value stored in the memory is used as an initial value. Usually, the servo system stores a learning value obtained by the learning operation in a DRAM (dynamic RAM) and uses the learning value in the subsequent servo control operation (head positioning control). The DRAM clears the stored learning value when the drive turns off.
In the disk drive having a servo system, a learning operation is performed whenever the spindle motor starts, so that a data read/write operation is always on standby until the learning operation ends. Consequently, the starting time of the drive (time for preparing the read/write operation) is relatively long. Disk drives have recently been widely used for mobile information equipment having a communications function as well as personal computers. The reduction of starting time is a challenge to improve the performance of the disk drives.
Conventionally, if a proper learning value is not obtained by a learning operation when a disk drive starts, the initial learning value calculated when the drive is manufactured is used. However, due to a change of use environment (particularly temperature) of disk drives, an amount of eccentricity of a disk may be changed after the disk drive is manufactured. The use of the initial learning value causes the drive to malfunction when it starts.
An object of the present invention is to shorten time for performing a learning operation related to disk runout compensation and thus shorten time for preparing a read/write operation.
According to an aspect of the present invention, a disk drive has a learning function of determining whether an initial learning value calculated by a learning operation when the drive is manufactured is valid or not when a spindle motor of the drive starts and omitting a learning operation when the initial learning value is valid.
More specifically, the disk drive comprises a disk on which servo data is prerecorded, a head configured to read/write data, an actuator mounted with the head and configured to locate the head in a target position on the disk, a servo controller configured to perform a seek operation and a tracking operation and locate the head in the target position on the disk, a first memory configured to store an initial learning value, a learning controller configured to stop the learning operation when the initial learning value is valid and perform the learning operation when the learning value is invalid, and a second memory configured to store a learning value calculated by the learning operation of the learning controller.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.